Hot water heaters monitor water temperature to determine when water should be heated to maintain a selected water temperature level. Heaters incorporating heat pumps to heat the water energize and de-energize a heat pump based on a measured temperature. If the temperature falls below a selected threshold, the heat pump may be energized to reheat the water. When demand for hot water drops, the heat pump may be de-energized. Operation of the heat pump should accurately track hot water demand to ensure maximum heating efficiency.
Water in the tank tends to stratify, with hot water at the top of the tank near a hot water outlet pipe and cold water at the bottom of the tank near a cold water inlet pipe. Water heated by the heat pump is deposited at the top of the tank, providing additional water that can be output via the output pipe. Thermometers may be placed in the outlet pipe, the inlet pipe, and/or a water pump that sends water to the heat pump to determine whether to energize the heat pump, but the stratification of the water in the tank makes it difficult for the temperature reading to accurately reflect the water temperature in the tank itself through temperature measurements in the pipes. Although it is possible to circulate the water through the de-energized heat pump and the tank to eliminate the stratification before measuring temperature, this would send cold water to the hot water at the top of the tank, undesirably lowering the overall water temperature and potentially requiring the heat pump to energize even though there originally may have been enough hot water at the top of the tank to meet demand. Because of this, any disturbance in the stratification of water in the tank is considered undesirable.
It is possible to place a temperature sensor at the hot water outlet pipe itself because this temperature would reflect the water that will be output to a user. However, if there is no demand for hot water for an extended period of time, the water in the tank may be cooler than the water in the outlet pipe. While the heat pump may be energized as soon as the water flowing through the outlet pipe reflects the lowered temperature of the water in the tank, the large amount of water in the tank causes a long time delay between the time the temperature drop is detected and the time the water is hot enough to use. Thus, currently known systems are unable to provide a temperature reading that is relevant enough to the temperature of usable hot water in the tank to accurately indicate whether the heat pump should be energized.
There is a desire for a system that can provide relevant, accurate temperature information for determining whether to energize a heat pump, improving energy efficiency.